An organic light emitting diode (OLED) refers to ‘a spontaneously light-emitting organic material’ that emits light by using electroluminescence where a fluorescent organic compound is allowed to emit light when an electric current is applied to the fluorescent organic compound. This OLED may be driven at a relatively low voltage and manufactured with a small thickness, and has a wide viewing angle and a swift response time as well. Therefore, the OLED has advantages in that, unlike the liquid crystal displays (LCD), it has an unchanged image quality and no image sticking even when seen right by the side, and may realize its full coloration as well. Therefore, the OLED has a high potential as one of leading devices of the next-generation flat panel display.
Such an OLED is generally formed by sequentially stacking an anode (ITO layer), an electron injection layer, an electron transport layer, an emissive layer, a hole transport layer, a hole injecting layer and a cathode on a transparent substrate. Here, electrons start to move when a voltage is applied to the OLED. In this case, in the cathode, the electrons move to the emissive layer by the aid of the electron transport layer. On the while, in the anode, holes from which electrons are escaped move to the emissive layer by the aid of the hole transport layer. When the electrons and the holes run into each other at the organic emissive layer, they are combined to form excitons having a high energy potential. The excitons emit light while dropping to a low energy level.
In order to facilitate the injection of electrons and improve the luminous efficiency, metals such as magnesium, magnesium-silver alloy, aluminum, lithium aluminum alloy and calcium have been generally used in OLED to form the cathode. However, when light is incident from the outside of the OLED, some of the incident light is reflected on the metallic cathode since the metallic cathode has a high surface reflectance. This internal reflection causes problems associated with the degradations in contrast and visibility of the OLED.
Therefore, a circular polarizer including a linear polarizer and a ¼ retardation plate has been used in the art to compensate for this degraded contrast of the OLED. However, the conventional circular polarizer has a problem in that, although it is used to improve the contrast, its transmittance is dropped by below 45% due to the absorption of light by the circular polarizer, which leads to the significantly degraded brightness of the OLED.
In the conventional OLED devices, leading factors that degrade the brightness of the OLED may be the total internal reflection caused by the difference in refractive indexes of respective layers constituting the OLED, the polarization (light absorption) by the circular polarizer, etc. Light emitted from the emissive layer of the OLED may be reduced in brightness by the reflection (i.e. total internal reflection) while being passed through the hole transport layer, the hole injecting layer, the anode and the transparent substrate, and be finally reduced in brightness by 10% or less while being passed through the circular polarizer, which leads to low light emission efficiency.
In order to solve these problems regarding the degraded brightness, a method of optically modifying a transmission path inwards the OLED device or using special substances at respective layers has been under consideration, but this alternative method has problems in that it is difficult to be put to practical use due to the inefficient process and low yield, and thus due to high manufacturing costs.